Is absolutely pure water, meaning water with no dissolved ions or minerals whatsoever, a conductor of electricity? Considering that tap water, which contains impurities, does conduct electricity, I want to specifically understand the role of these dissolved substances. Does H2O itself, the molecule, facilitate any electron flow or ion movement under an electric field, or is the conductivity solely dependent on the presence of dissolved ions like sodium, chloride, etc.? Furthermore, if pure water has a very low conductivity, can we consider it an insulator for practical purposes? And what are the implications for experimental setups requiring extremely high resistance or isolation?

Answer

Pure water, ideally, is a very poor conductor of electricity. Conductivity arises from the presence of mobile ions that can carry charge. In perfectly pure water (H₂O), there are very few ions present. Some water molecules do dissociate into hydrogen ions (H⁺) and hydroxide ions (OH⁻), according to the equilibrium:

H₂O ⇌ H⁺ + OH⁻

However, the extent of this autoionization is very limited. At room temperature (around 25°C), the concentration of both H⁺ and OH⁻ ions in pure water is only about 10⁻⁷ moles per liter. This very low ion concentration means that pure water has a very high electrical resistivity (around 18.2 megohm-cm), indicating extremely poor conductivity.

Why Isn’t it a Perfect Insulator?

Even with such low ion concentrations, pure water isn’t a perfect insulator. The small number of ions present do allow for a very small amount of current to flow under an applied electric field.

Factors Affecting Water Conductivity:

In reality, it is practically impossible to obtain and maintain truly pure water. Water’s excellent solvent properties mean it readily dissolves various substances, including atmospheric gases (like carbon dioxide) and minerals from containers and pipes. These dissolved substances dissociate into ions, dramatically increasing the water’s conductivity. Even trace amounts of impurities can significantly increase conductivity.

• Dissolved Salts: Salts like sodium chloride (NaCl) dissociate into sodium ions (Na⁺) and chloride ions (Cl⁻) when dissolved in water, providing a large number of charge carriers.
• Acids and Bases: Acids (like hydrochloric acid, HCl) increase the concentration of hydrogen ions (H⁺), and bases (like sodium hydroxide, NaOH) increase the concentration of hydroxide ions (OH⁻), both leading to higher conductivity.
• Dissolved Gases: Carbon dioxide (CO₂) dissolves in water to form carbonic acid (H₂CO₃), which then dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻), also increasing conductivity.
• Temperature: The autoionization of water is temperature-dependent. As temperature increases, the equilibrium shifts towards higher concentrations of H⁺ and OH⁻ ions, leading to increased conductivity.

Practical Implications:

• Electrical Safety: Because tap water, well water, and even most distilled water contain dissolved ions, they are conductors of electricity. This is why it’s extremely dangerous to use electrical appliances near water sources; the water can provide a pathway for electricity to flow through the body, leading to electric shock.
• Water Quality Monitoring: Electrical conductivity is used as a measure of water purity. Higher conductivity indicates higher levels of dissolved ions, suggesting the presence of impurities or pollutants.
• Laboratory Applications: In some sensitive laboratory experiments, very pure water (often produced by reverse osmosis, deionization, and distillation) is required to minimize interference from ionic impurities.